The instant invention relates to an interlaminar sandwich of the type wherein a diffraction structure is embedded between two lacquer layers, and wherein a heat activated layer bonds the interlaminar sandwich to a substrate. The invention also relates to a method of utilizing the interlaminar sandwich to protect an identification document.
Such an interlaminar sandwich is advantageously used to increase forgery protection of documents of all kinds, whereby the interlaminar sandwich with its eye-catching optically variable patterns is pasted on the document.
A multilayer structure of hot-embossed films of various designs with holograms is known from EP 170 832 A1 (U.S. Pat. No. 4,629,646) for example. Such structures are characterized by an interlaminar sandwich comprising a separation layer, a lacquer layer containing the hologram, and an adhesive layer. The separation layer connects the interlaminar sandwich to a support film until it is pasted on a substrate. The adhesive layer is heated through the support film, the separation layer and the lacquer layer, and a high degree of adhesion to the substrate is achieved. Following the gluing operation, the separation layer makes it possible for the support film to be easily detached from the interlaminar sandwich which is now firmly bonded to the substrate.
A five-layer film with two lacquer layers whose optical properties are clearly distinct from each other and which constitute a decorative layer is known from DE-OS 35 27 412 (U.S. Pat. No. 4,17,615).
On identification cards the individual layers are often glued in a grid so that one or several thin intermediary layers tear at predetermined points in case of an unwanted separation of the protective layers. EP-253 089 A1 described this method for the protection of optical markings, while the protection of an identification document made of paper and laminated between two films can be increased according to claim U.S. Pat. No. 4,057,919.
Different embossing processes for low-cost duplication of optical diffraction structures in thermoplastic surfaces are described, for example, in CH-PS (Swiss patent) 661 683 (U.S. Pat. No. 4,556,378), CH-PS 595 664 (U.S. Pat. No. 4,250,217) and CH-PS 594 936 (U.S. Pat. No. 4,184,700).
It is the object of the instant invention to create an interlaminar sandwich having optical diffraction structures embedded therein, which interlaminar sandwich is pasted on a substrate under the effect of heat and whose diffraction structures are certain to be destroyed when an attempt is made to detach the interlaminar sandwich from the document.